The invention relates to a measurement amplification device for detecting the detuning of a measuring bridge to which a bipolar, rectangular supply voltage is supplied.
The invention also relates to a measurement amplification method for detecting the detuning of a measuring bridge to which a bipolar, rectangular supply voltage is supplied.
Known devices have an integrating A/D converter which generates a digital output value which is representative of the bridge detuning, based on an analog bipolar detuning signal from the measuring bridge. The A/D converter comprises an integrator which integrates a measured signal that is constantly applied to said integrator during operation and a working level of a reference signal that is applied to said integrator at certain times. The A/D converter also comprises a comparator which is connected downstream of the integrator and compares an integrator output signal with a threshold value. The comparator determines each time point at which the integrator output signal crosses the threshold value. By means of a comparator output signal which is fed back to a switchable reference voltage source, the comparator controls the application, at certain times, of the working level of the reference signal. Time measuring means detect the duration of each measured interval, since it is thereby defined that throughout its duration, the working level of the reference signal is applied to the integrator as the basis of a measure for the bridge detuning.
Methods and devices of this type for digitizing detection of measured signals, in particular detuning signals of a measuring bridge, are known in the context of carrier frequency amplification and carrier frequency conversion. The term “measuring bridge” can be understood broadly and includes not only the classical Wheatstone bridge, but also “half-bridges” and other circuits fed with a supply voltage, having at least one variable resistor, the adjustment of which leads to detuning of a detuning voltage detectable from the circuit.
Before the introduction of the carrier frequency principle, a DC voltage measured signal, for example, the detuning signal of a measuring bridge was pre-amplified by means of a DC amplifier and then digitized in an integrating A/D converter.
The principle of the integrating A/D converter has long been known in many variants, for example, from DE 21 14 141, DE 28 20 601 C2 and DE 100 40 373 A1. In an integrating A/D converter, the measured signal is applied to an input of an operational amplifier configured as an integrator. For configuration as an integrator, the output of the operational amplifier is linked via a capacitor to its measured signal input. Also linked to the measured signal input of the operational amplifier is the feed line for a DC voltage reference signal. This reference signal is applied at a working level only at certain times. During the remainder of the time, it is applied at a quantitatively lower rest level or is completely disconnected from the input of the operational amplifier. During a first pulse portion of a measuring clock pulse, during which the working level of the reference signal is not applied, the capacitor is charged up by the measured signal amplified in the operational amplifier. If, after a pre-determined time period, the working level of the reference signal is switched on, the capacitor discharges during a second clock pulse portion, so that the integrator output signal declines. The time point of a zero-crossing or, more generally, a threshold-crossing by the integrator output signal is detected by means of a comparator connected downstream, which itself initiates, via control means, the disconnection of the working level of the reference signal from the integrator input, so that a new measuring clock pulse can begin with the charging of the capacitor. The duration of the second clock pulse portion, that is, the time span during which the working level of the reference signal is applied to the integrator, is measured with suitable time measuring means, for example, a clocked counter. The measured duration, referred to herein as the measured interval, represents a measure for the charging of the capacitor taking place in the first clock pulse portion, and therefore for the level of the measured signal. In the case of time measurement by means of a clocked counter, the counter value can be used directly as a digital measure for the measured signal, i.e., in particular the detuning signal of the measuring bridge.
In the past, precise pre-amplification of the DC measured signal presented difficulties due to the associated offset voltages and their drift. This was partially corrected by the introduction of the carrier frequency principle, which also brings advantages with regard to the suppression of 1/f noise. With this principle, the measuring bridge is fed with a bipolar rectangular signal. This results in an also bipolar, rectangular detuning signal of the measuring bridge. This can be pre-amplified with known AC amplifiers. The pre-amplified AC signal can then be converted by in-phase rectification and subsequent low-pass filtration into a DC measured signal. This is known from DE 2164315. The subsequent digitization of the DC measured signal can be performed as described above.
As an alternative to in-phase rectification and subsequent low-pass filtration, a system is known from WO 03/087749 A2 wherein the pre-amplified AC detuning signal is sampled directly at high frequency, wherein the difference between the mean values of the sampled values detected in respective successive half-periods of the bipolar detuning signal is used as a measure for the bridge detuning. Compared with the previously described method of in-phase rectification and subsequent low-pass filtration, this method has the advantage of digital elimination of the offset and drift from the AC pre-amplification. However, it has the disadvantage of needing a very high frequency sampler, which has a limited resolution.
A device with voltage/frequency conversion of a detuning signal of a measuring bridge is known from DE 36 33 790 A1, wherein according to the principle of alternating integration and de-integration of the detuning signal and comparison with a threshold value in a comparator, a pulse train which has a frequency that is representative of the bridge detuning is generated. In the known device, the polarity of the supply voltage of the measuring bridge is arbitrarily reversed according to an external polarity-reversal clock pulse. The detuning signal applied to the measuring bridge also changes its polarity accordingly, and this normally takes place during an integration phase, compared to the duration of which the duration of the de-integration phase is negligibly short. In order to avoid a consequently falsely displaced pulse in the pulse train, a charge balance cycle which causes a delay of the pulse to be output is set in motion, and this also initiates the next de-integration phase. This device has several disadvantages. Firstly, even the generation of a pulse train, the frequency of which must be measured for the calculation of a measured voltage value, is unfavorable, because this is technically complex and long-winded, since for exact measurement of a frequency, many pulses, that is, many integration and de-integration phases must be taken into consideration. Secondly, the principle is based on neglecting offset voltages, capacitor residual voltages and the time needed for the de-integration. In many cases, these may be realistic assumptions. But where very rapid and/or accurate measurement is required, such factors can no longer be ignored, so that the known measuring method and the corresponding device cannot be used. Thirdly, the charge balance cycle requires a large number of additional components, which implies a correspondingly greater technical effort and energy consumption.
From DE 42 22 580 A1, a conventional A/D converter circuit with an integrator is known, to which, apart from the measured signal, different levels of a reference signal are alternately applied.
U.S. Pat. No. 4,031,532 discloses an A/D converter circuit for conversion of a detuning voltage of a measuring bridge. The circuit has an integrator to which, apart from the detuning signal, a reference signal derived from the bridge supply voltage is applied, the polarity of said reference signal being switched over for integration and de-integration. The polarity of the supply voltage, however, remains constant.